To get the key you need, measure the shaft the key is to go on to, then use this chart to get the key size # you need. Please do not call asking what key your clock needs, as you will be told it would still have to be measured. Again, you are measuring the shaft that the key goes onto in MM and then using the below chart.
1 CM = 10 MM
25.4 MM (2.54 CM) = 1 inch
Most modern German wall or table clocks (post 1960) takes number 8 or 4 key